David Cameron and his former director of communications, Andy Coulson - who it emerges was operating under only limited security clearance while working at No 10. Photograph: David Fisher/Rex Features

It was the £140,000-a-year job that placed Andy Coulson at the heart of British power. But the former News of the World editor's predecessors as prime ministerial spin doctor were stumped as to how he could have done the job with only limited security clearance befitting a far more junior civil servant.

As the Cabinet Office faced growing pressure to explain why Coulson had not been cleared to the same security level as other recent Downing Street advisers, former No 10 staff told how they could not have operated on issues ranging from Afghanistan to the economy without the rigorous "developed vetting" process they underwent. This was aimed at uncovering lies and anything that could make an official susceptible to blackmail.

Lance Price, Tony Blair's former deputy political spokesman, said Coulson's lower level of security clearance would have made it almost impossible to advise on issues including Nato,European security, Afghanistan, the terror threat to the UK and the situation in Northern Ireland.

"I find it breathtaking that the director of communications would have anything less than the full level of security, because in that kind of job you have to be able to see and assess just about everything that passed the prime minister's desk in terms of communications strategy and how it might impact," Price said.

"I would see papers relating to negotiations in Northern Ireland which were pretty sensitive, I attended private meetings with Tony Blair in Washington with Bill Clinton and the secretary of state for defence. The implication of him not having the highest level of vetting is that there would have been quite a lot of papers he wouldn't have been able to see. Even in my work, it would have been extremely difficult to have done the job properly and I was in a less senior position than Coulson."

Alastair Campbell, Blair's former press secretary, said it was "quite odd" that Coulson was not vetted at a higher level, explaining how he was only able to read some of the most sensitive material passing through No 10 because of rigorous vetting that Coulson did not undertake.

"Essentially it was understood that Jonathan Powell [Blair's chief of staff] and I, in addition to other senior civil servants, were able to attend any meeting and see any papers that went to the prime minister. It is very hard to see how you could do the press and strategy job, particularly on foreign affairs, without being fully in the picture. You had to be trusted with all the information and then know how much you were entitled to divulge.

"For example, I don't see how Coulson could have attended Cameron's meeting with President Obama in Washington in 2010, when they discussed Afghanistan, the Middle East, the Gulf of Mexico oil spill and the economy, without developed vetting. A lot of the material about the economy, one of the biggest issues of the day, also required developed vetting."

Coulson was cleared to "security check" level, a standard that grants regular access to material classified as secret, but only "occasional, controlled access" to top-secret documents. Roles involving unsupervised access to top-secret material require higher-level developed vetting, according to official guidance. It involves an extra questionnaire, criminal record, security services and credit reference checks, and an extended, typically three-hour interview, plus reference checks by phone or in person.

"We have to look at your loyalty, honesty and reliability, and whether you could be particularly vulnerable to bribery or blackmail," the guidance states. "We will question you about your wider family background (relationships and influences), past experiences (if any) of drug taking, financial affairs, general political views (though not which party you support), hobbies, foreign travel and so on."

A senior former Downing Street official said: "The grilling you receive is intense, highly detailed and incredibly personal – for example, going through a very long list of sexual practices and being asked what you had and hadn't done. Answer yes to any one of them and you'd be required to give full details: when, with whom, how recently, and so on.

"If one incident stands out on your employment or financial or family record, you could spend up to an hour under intense scrutiny about that one thing. They then go and interview at least one of your closest friends to see if their answers tally up with yours. It's impossible to 'prep' your friends for their interviews because you can't remember everything you said. Your only option is to tell the complete truth to the interviewer and tell your friends to do the same."

John McTernan, Blair's former political secretary, said that when he was subject to developed vetting, investigators asked his friend: "Have you seen Mr McTernan drunk? How do you think he manages his money? Does he gamble? And when he's drunk, what's he like with his children?"

McTernan wrote in the Daily Telegraph: "Did they really not probe the possibility that Coulson's past might impact on his proposed role? It seems so unlikely. Did the PM, or his team, do no due diligence on Coulson?"

Vetting levels

Strap' classification

Used for particularly sensitive information, and is sub-divided into three levels of access: documents have a cover sheet that must be signed each time they are accessed.

Developed vetting (DV)

Required for any officials with routine or unsupervised access to top-secret material. To receive this level of clearance, applicants must complete a 53-page assessment form. That is then verified with a three-hour interview, and references are cross-checked. The process can be expedited to a few weeks, but can take up to six months.

Security check (SC)

Grants routine access to secret material, but only occasional supervised access to top-secret documents. Applicants fill out a 29-page form and are subject to security and credit reference checks. The process typically takes just a few weeks, but can be completed faster.

Counter-terrorism check (CTC)

Basic security check for people in close proximity to public figures, or with access to low-level sensitive information. Applicants have their criminal records and other security information checked.